This disclosure relates generally to the field of optical particle control and more particularly to the field of optical dielectric particle control and manipulation.
Optical tweezers (otherwise known as “single-beam gradient force traps”) are scientific instruments that use a highly-focused laser beam to provide an attractive or repulsive force (typically on the order of piconewtons), to physically hold and move microscopic dielectric objects. Optical tweezers have recently been particularly useful in a variety of biological studies.
Optical tweezers are capable of manipulating nanometer and micrometer-sized dielectric particles via extremely small forces of a highly focused laser beam. The beam is typically focused through a microscope objective lens. The narrowest point of the focused beam, known as the beam waist, contains a very strong electric field gradient. Dielectric particles are attracted along the gradient to the region of strongest electric field, which in this case is the center of the focused beam. The laser light also tends to apply a force on particles in the beam along the direction of beam propagation. This force can be explained and envisioned when light is considered to be a group of particles, wherein each of the light particles impinges on the tiny dielectric particle in its path. Such an interaction is known as the scattering force and results in the dielectric particle being displaced slightly downstream from the exact position of the beam waist.
Optical tweezers are very sensitive instruments and are capable of the manipulation and detection of sub-nanometer displacements for sub-micrometer dielectric particles. For this reason, they are often used to manipulate and study single molecules wherein the molecule is typically bonded to a bead attached to the molecule. Deoxyribo Nucleic Acid (DNA) and the proteins and enzymes that interact with it are commonly studied in this way.
Proper explanation of optical trapping behavior depends upon the wavelength of light used in the trapping compared to the size of the particle to be trapped. In cases where the wavelength is much smaller than the dimensions of the particle, a simple ray optics treatment is sufficient. If the wavelength of light far exceeds the particle dimensions, the particles can be treated as electric dipoles in an electric field. For optical trapping of dielectric objects of dimensions within an order of magnitude of the trapping beam wavelength, the only accurate models involve the treatment of either time dependent or time harmonic Maxwell equations using appropriate boundary conditions.
As indicated, in cases where the wavelength of light is significantly smaller than the diameter of a trapped particle, the trapping phenomenon can be explained using ray optics. Individual rays of light emitted from the laser will be refracted as it enters and exits a dielectric particle. As a result, the ray will exit in a direction different from that of its origin. Since light has momentum associated with it, the change in the direction of the light indicates that the momentum of the light has changed. Newton's third law dictates that there should be an equal and opposite momentum change on the particle.
Most optical traps operate with a Gaussian beam (TEM00 mode) profile intensity. If the particle is displaced from the center of the beam, the particle has a net force returning it to the center of the trap. A more intense beam imparts a larger momentum change towards the center of the trap while a lesser intense beam imparts a smaller momentum change away from the trap center. In either case, the net momentum change, or force, returns the particle to the trap center.
If a particle is located at the center of the beam, then individual rays of light are refracting through the particle symmetrically, resulting in no net lateral force. The net force in this case is along the axial direction of the trap, which cancels out the scattering force of the laser light. The cancellation of the axial gradient force with the scattering force is what causes the particle-bead to be stably trapped slightly downstream of the beam waist.
Prior art techniques of controlling and manipulating minute particles are generally limited to single a particle or are made much more complex to enable multiple particle use. Additionally, common optical tweezing operations become unmanageable when it is desired to lift particles over walls or barriers, resulting either in an inability to adequately manipulate the particles or a loss of the particle itself. It is thus desirable to have particle manipulation capability that readily allows control and movement of multiple particles, does so with minimal complexity, and in addition provides relative ease and efficiency in the three dimensional manipulation and control of particles over obstructions.